As it is known, motor vehicles are generally provided with air-conditioning systems of the type comprising, in succession, a compressor, a condenser, an expansion valve, and an evaporator, connected to one another via a duct through which a coolant, generally Freon, flows.
As refrigerating fluids (or coolants) use is typically made of fluids such as ammonia, methyl chloride, sulphurous anhydride; halogenated hydrocarbons such as Freon (for example R11, R12, R114, R134a), or, further, substances like carbon dioxide and hydrocarbons like propane.
In particular, air-conditioning systems of the type described above are generally configured to implement a sub-critical refrigerating cycle of the type illustrated in FIG. 1A, i.e. a refrigerating cycle wherein the maximum pressure at which the refrigerating fluid is compressed and to which reference is generally made as condensation pressure, is always below the critical pressure pc characteristic of the refrigerating fluid itself. Under those conditions, as a matter of fact, along the high-pressure tract of the cycle condensation of the refrigerating fluid occurs, the refrigerating fluid thereby yielding heat to the outside, i.e. a thermal exchange takes place between a condensing fluid (which shall be partly in the liquid state and partly in the vapour state) and a gas (generally air). Air-conditioning systems are further provided with a fan which is arranged downstream of the evaporator and is operated to push conditioned air into the passenger compartment of the motor vehicle, and a fan which is arranged in front of the condenser and is operated automatically to maintain the coolant pressure in the condenser (condensation pressure) at optimal values in all operating conditions. Operation of the condenser fan determines, in fact, an increase in the air flowrate impinging upon the condenser, thus determining a reduction of the condensation pressure.
The condenser fan is operated both for reasons of safety, i.e., to prevent the coolant pressure in the condenser from reaching the tightness pressure of the pipes in the air-conditioning system in which the coolant flows, and to maintain an acceptable performance of the air-conditioning system. A condensation pressure that is excessively high, in fact, causes a consequent increase in the coolant pressure in the evaporator and, hence, also of the conditioned air temperature at the evaporator outlet.
Typically, the switching-on, switching-off, and the rotation speed of the condenser fan are controlled via a pressure switch calibrated on a number of threshold values of the condensation pressure. When each of these threshold values is exceeded, the condenser fan is operated at a corresponding rotation speed. In the majority of air-conditioning systems of modern motor vehicles, the condenser fan is operated at two levels, so that when a lower threshold value is exceeded the condenser fan is driven at a lower rotation speed, whereas when a higher threshold value is exceeded the condenser fan is driven at a higher rotation speed.
In order to enable efficient energy management of the air-conditioning system, in more recent models of motor vehicles, the pressure switch is increasingly more frequently replaced by a linear pressure sensor, whilst the condenser fan is controlled with PWM (Pulse Width Modulation) techniques.
Such a control of the condenser fan entails, however, the disadvantage of not taking into account the impact that the use of the condenser fan has on the overall energy balance of the motor vehicle. As is known, in fact, the compressor behaves like a pump that operates between two pressure levels and, consequently, it may be readily appreciated how the more the coolant pressure at the evaporator output approaches the coolant pressure at the condenser inlet, the smaller the work that the compressor has to do to bring the coolant to the required pressure and, consequently, the smaller the mechanical power that the compressor absorbs from the internal combustion engine.
In fact, even though operation of the condenser fan for reducing the condensation pressure results in a reduction of the work that the compressor has to perform to bring the coolant pressure to the required value, operation of the condenser fan results, on the other hand, in an increase in the electric power absorbed from the alternator, and, since the latter is also operated by the internal combustion engine via a belt, a consequent increase in the mechanical power absorbs from the internal combustion engine and hence in the fuel consumption.
US patent application No. 2007/0125106 describes instead an automotive air-conditioning system configured to implement a super-critical refrigerating cycle of the type illustrated with a dotted line in FIG. 1B, i.e. a refrigerating cycle wherein the refrigerating fluid, having been compressed to a pressure greater than its critical pressure pc, does not condense and, along the high-pressure tract of the cycle, a thermal exchange between two gases occurs. As a consequence, differently from an air-conditioning system configured to implement a sub-critical refrigerating cycle, an air-conditioning system configured to implement a super-critical refrigerating cycle provides for the use of a radiator (i.e. a gas/gas heat exchanger) instead of a condenser.
In particular, the above-mentioned patent application teaches controlling, for the super-critical refrigerating cycle, the switching-on and the rotational speed of the radiator fan as a function of the difference ΔT between the refrigerant fluid temperature (Tg) leaving the radiator and the atmospheric temperature (Ta). Alternatively, still for the operation under super-critical conditions, the patent application cited above teaches controlling the switching-on and the rotational speed of the radiator fan as a function of the difference between the actual high pressure (Ph) as measured along the super-critical tract of the refrigerating cycle, and a target pressure (Pset) which is, in turn, a function of atmospheric temperature Ta.
For the condition wherein actual high pressure Ph is less than the critical pressure of the refrigerating fluid and the radiator operates as a condenser, the patent application referred to above teaches instead a radically different approach for controlling the switching-on and the rotational speed of the fan. In particular, for sub-critical operating conditions, there is suggested that condensation pressure Ph be not controlled, and that, at one time:                a) the super-cooling degree of the refrigerating fluid leaving the radiator be controlled and thereby kept within a predetermined range, by controlling the opening degree of an electronically controlled expansion valve, so as to enhance a coefficient of performance (COP) of the system; and        b) the cooling capacity of the radiator be controlled by varying the fan speed in response to variations in the condensation pressure Ph.        
This solution involves a major computational load for the electronic control unit of the air-conditioning system, since the latter has to implement both a control algorithm of the expansion valve and a control algorithm of radiator fan.